Pure Appl. Geophys. Ó 2014 Springer Basel DOI 10.1007/s00024-014-0896-6 Pure and Applied Geophysics

Composition, Alteration, and Texture of Fault-Related Rocks from Safod Core and Surface Outcrop Analogs: Evidence for Deformation Processes and Fluid-Rock Interactions

1 1 1 1 1 KELLY K. BRADBURY, COLTER R. DAVIS, JOHN W. SHERVAIS, SUSANNE U. JANECKE, and JAMES P. EVANS

Abstract—We examine the fine-scale variations in mineralogi- 1. Introduction cal composition, geochemical alteration, and texture of the fault- related rocks from the Phase 3 whole-rock core sampled between 3,187.4 and 3,301.4 m measured depth within the San Andreas Fault Well-constrained geological, geochemical, and Observatory at Depth (SAFOD) borehole near Parkfield, California. geophysical models of active fault zones are needed if This work provides insight into the physical and chemical properties, we are to understand fault zone behavior and earth- structural architecture, and fluid-rock interactions associated with the actively deforming traces of the San Andreas Fault zone at depth. quake deformation, constraining the factors that affect Exhumed outcrops within the SAF system comprised of serpentinite- the distribution of earthquakes, and the nature of slip bearing protolith are examined for comparison at San Simeon, Goat in the shallow crust by developing realistic models of Rock State Park, and Nelson Creek, California. In the Phase 3 SAFOD drillcore samples, the fault-related rocks consist of multiple subsurface fault zone structure and ground motion juxtaposed lenses of sheared, foliated siltstone and shale with block- predictions. Earthquakes nucleate in rocks at depth in-matrix fabric, black cataclasite to ultracataclasite, and sheared (e.g., FAGERENG and TOY 2011;SIBSON 1977; 2003), serpentinite-bearing, finely foliated fault gouge. Meters-wide zones of sheared rock and fault gouge correlate to the sites of active yet until recently (see ANDO, 2001;BOULLIER et al. borehole casing deformation and are characterized by scaly clay 2009;CORNET et al. 2004;HEERMANCE et al. 2003; fabric with multiple discrete slip surfaces or anastomosing shear HICKMAN et al. 2004;HICKMAN et al. 2007;HIRONO zones that surround conglobulated or rounded clasts of compacted clay and/or serpentinite. The fine gouge matrix is composed of Mg- et al. 2007;OHTANI et al. 2000;TANAKA et al. 2002; rich clays and serpentine minerals (saponite ± palygorskite, and TOBIN and KINOSHITA 2006;TOWNEND et al. 2009;ZO- lizardite ± chrysotile). Whole-rock geochemistry data show BACK et al. 2007;ZOBACK et al. 2010), we have only increases in Fe-, Mg-, Ni-, and Cr-oxides and hydroxides, Fe-sul- fides, and C-rich material, with a total organic content of [1% had limited sampling of rocks or observations from locally in the fault-related rocks. The faults sampled in the field are within active plate-boundary fault zones where composed of meters-thick zones of cohesive to non-cohesive, ser- earthquake nucleation and rupture propagation has pentinite-bearing foliated clay gouge and black fine-grained fault rock derived from sheared Franciscan Formation or serpentinized occurred. Direct knowledge of rock properties from Coast Range Ophiolite. X-ray diffraction of outcrop samples shows active fault zones through integrated drilling, field, that the foliated clay gouge is composed primarily of saponite and and laboratory studies can provide significant insight serpentinite, with localized increases in Ni- and Cr-oxides and C-rich material over several meters. Mesoscopic and microscopic textures into fault processes, and can be used to: (1) identify and deformation mechanisms interpreted from the outcrop sites are systematic compositional and textural changes to infer remarkably similar to those observed in the SAFOD core. Micro- the chemical and physical processes involved in fault scale to meso-scale fabrics observed in the SAFOD core exhibit textural characteristics that are common in deformed serpentinites zone evolution; (2) delineate the dimensions of the and are often attributed to aseismic deformation with episodic structural and permeability fault zone architecture; seismic slip. The mineralogy and whole-rock geochemistry results and (3) consider the role of fluid migration and fluid- indicate that the fault zone experienced transient fluid–rock inter- actions with fluids of varying chemical composition, including related alteration throughout deformation (BRODSKY evidence for highly reducing, hydrocarbon-bearing fluids. et al. 2010; CAINE et al. 1996, 2010;CHESTER and LOGAN 1986;COWAN 1999;EVANS and CHESTER 1995; FAGERENG and SIBSON 2010;KNIPE et al. 1998;MARONE and RICHARDSON 2010;MENEGHINI and MOORE 2007; ROWE et al. 2009;SHIPTON AND COWIE and COWIE 2001; 1 Geology Department, Utah State University, Logan, UT 84322-4505, USA. E-mail: [email protected] SIBSON 1989;WIBBERLEY et al. 2008). K. K. Bradbury et al. Pure Appl. Geophys.

The San Andreas Fault Observatory at Depth ZOBACK et al. 2011). Previous research by BRADBURY (SAFOD; HICKMAN et al. 2004;HICKMAN et al. 2007; et al.(2007), BRADBURY et al.(2011), HOLDSWORTH ZOBACK et al. 2010; http://www.earthscope.org/ et al.(2011), SPRINGER et al. (2007), and THAYER and observatories/safod) near Parkfield, California, pro- ARROWSMITH (2005) illustrate the geologic and vides a nearly continuous record of rock cuttings and structural setting and provide detailed core observa- a limited amount of core from the subsurface through tions of the SAFOD site. the Pacific Plate northeastward across the San Serpentinite was identified in several SAFOD Andreas Fault and into the North American Plate samples, between a depth of 3 and 4 km (BRADBURY (Fig. 1). At SAFOD, in situ fault-related rocks from et al. 2011;MOORE and RYMER 2007;MOORE and *3 km depth were sampled along the central RYMER 2012;SOLUM et al. 2006; Phase 3 Core Photo creeping segment of the San Andreas Fault (SAF). Atlas, http://www.earthscope.org/observatories/ The surface creep rate of the SAF near SAFOD is 20 safod). This mineralogy and the associated alter- mm/yr (TITUS et al. 2006), and is interpreted to occur ation products may promote weak fault behavior on multiple parallel strands in the subsurface (HOLE (HIRTH and GUILLOT 2013;MOORE et al. 1997;MOORE et al. 2006;MCPHEE et al. 2004;THURBER et al. 2004; and RYMER 2012;REINEN et al. 2000; SCHLEICHER et al. THURBER et al. 2006;ZOBACK et al. 2005). Within the 2012). In the two *1–2 m thick gouge zones asso- SAF here, a series of repeating microearthquakes ciated with active creep demonstrated by borehole occur near 2,500–2,800 m vertical depth, or about casing deformation at SAFOD, serpentinite and ser- *50–300 m from the actively creeping fault strands pentinite-derived clays are present (SOLUM et al. intersected by the SAFOD borehole (Fig. 1;NADEAU 2006, 2007;BRADBURY et al. 2011;MOORE and RYMER et al. 2004;THURBER et al. 2004;ZOBACK et al. 2010; 2009, 2012). The two zones, referred to as the

True Vertical Depth Cross Section N 45 E (K.B., m) Geologic Plate PA Boundary - 2600

Hole G Runs (1,2,3) 3157 m MD - 2650 NA

North American Hole G Runs (4,5,6) Plate (NA) - 2700 Damage Zone Goat Rock - 2750 SDZ 3192 m MD CDZ 3413 m MD San Francisco 3302 m MD - 2800

Nelson Creek 1150 12001250 1300 1350 1400 SAFOD N 45 East (m) San Simeon San Andreas Fault

Pacific Plate (PA) Los Angeles

Figure 1 Location map of the SAFOD borehole and surface outcrops in California relative to the positions of the San Andreas Fault, Pacific Plate, and North American plate. The inset illustrates a simplified vertical profile of the SAFOD borehole, location of the targeted earthquakes, SDZ, CDZ, and the associated damage zone (JEPPSON et al. 2010;ZOBACK et al. 2010), and the Phase 3 core as sampled at depth (modified from Phase 3 Core Photo Atlas, http://www.earthscope.org) Evidence for Deformation Processes and Fluid-Rock Interactions

a i 3312.3 m

3190.4 m

b h

3192.9 m 3301.3 m

c g

3195.8 m 3297.7 m d e f

3197.0 m 3197.8 m 3297.5 m

Black cataclasite to Phyllosilicate-rich block-in-matrix melange ultracataclasite SDZ CDZ Phyllosilicate-rich block-in-matrix melange

~ 95 m Gap in Core a b c d efg h i

7 m m m m m 0 m 1 m 6 m 12. m m m .4 m 9.4 .9 m 5 m 1 m .6 33 8.3 9.8 .3 m 4.3 m 5.8 7.9 00 04. 05. 07. 08. 10. 329 33 33 3311 3186.7 318 318 3192.8 m3193 319 319 3197.43198.9 m 3199.5 m m 3294.9 3296m 329 33 3302.5 m 33 33 33

Extent of Low Velocity Zone (after Zoback et al., 2010) Location of thin-section photomicrographs shown in Figures 4,5, and 7 and Extent of gas rich zones (after Weirsberg & Erzinger, 2009) whole-rock geochemical analyses shown in Figures 4A & 4B Total Organic Carbon (TOC) sample results shown in Table 2 b Location of image shown above (may or may not correspond to sample location)

Figure 2 Mesoscopic textures of fault-related rocks in SAFOD Phase 3 core: a foliated block-in-matrix fabric within damaged rocks west of the SDZ; b localized black (carbon-rich) staining parallel to fracture surface at 3,192.9 m MD; c Black fault-related rock consists of cataclasite to ultracataclasite at 3,195.8 m MD; d serpentinite and scaly clay gouge of the SDZ at *3,197 m MD; e foliated fault gouge with scaly clay fabric at 3,197.8 m MD; f clay-rimmed altered serpentinite and siltstone clasts within foliated fault gouge of the CDZ at 3,297.5 m MD; g foliated fault gouge with scaly clay matrix and numerous rounded and lens-shaped, altered serpentinite clasts and fragments at 3,297.7 m MD; h block of intensely sheared siltstone with abundant calcite veinlets to the immediate northeast of the CDZ fault boundary; and i intensely sheared black clay (organic-rich?) interval with shiny, polished slickensided surfaces at 3,312.3 m MD. Core depths reported here are in meters measured depth along the main SAFOD borehole. Extent of low-velocity (after ZOBACK et al. 2010) and gas-rich zones (after WEIRSBURG and ERZINGER 2011) are shown for reference. Location of samples discussed in this paper are also illustrated

Southwest Deforming Zone (SDZ) at 3192 meters To provide a broader geological context for our measured depth and the Central Deforming Zone observations and analyses of SAFOD core (Fig. 2), (CDZ) at 3,302 m measured depth (after ZOBACK we sampled numerous outcrops within the central to et al. 2010), are surrounded by layers of cohesive northern SAF system (Fig. 3) that included shear black fault-related rock composed of cataclasite, zones in serpentinite or serpentinite-derived clays ultracataclasite, sheared siltstone, and/or mudstone (ALLEN 1968;IRWIN and BARNES 1975;MOORE and with block-in-matrix fabric (BRADBURY et al. 2011). RYMER 2012). Large-scale strike-slip faults that cut As observed in the cuttings (BRADBURY et al. 2007), serpentinite are common in regions where continental and supported by geophysical data (JEPPSON et al. transform faults develop after plate convergence 2010), additional faults and/or gouge zones may exist (MOLNAR and DAYEM 2010). This allows us to conduct in the zone between *2.5 and 4 km, and borehole core to outcrop-scale analyses of the evolution, deformation occurred at the CDZ (JEPPSON et al. mechanics, and strength of serpentinite-bearing faults 2010). that have implications for other strike-slip fault K. K. Bradbury et al. Pure Appl. Geophys.

a b

c

d e

SAF

f g E W SAF Te

30 cm Evidence for Deformation Processes and Fluid-Rock Interactions b Figure 3 et al. 2011;MOORE and RYMER 2007;MOORE and Images of surface outcrops sampled in this study: a view to north of RYMER 2012). In this paper, we also document the SAF-related steeply east-dipping shear zone in shear zone of serpentinite-bearing Franciscan rocks exposed at Goat Rock State presence of carbon-rich materials and distinct geo- Beach, California (UTM WGS 84 10S, 489085 E, 4254741 N); chemical alteration signatures adjacent to and within b foliated phyllosilicate-rich gouge from shear zone at Goat Rock the fault zone. These data provide evidence for the State Beach, California; c Block-in-matrix fabric exposed in sheared Franciscan me´lange north of San Simeon State Beach, role of fluids during deformation along the SAF near California (UTM WGS 84 10S, 0669047, 3941268 to 0668975, SAFOD. 3941321); d Nelson Creek outcrop of the San Andreas Fault, approximately 2.3 km north of the SAFOD drill site (UTM location 10S, WGS 84, 720794 E, 3986183 N), where an approximately 6 m wide outcrop view of a strand of the San Andreas fault (SAF) 2. Composition and Texture of SAFOD juxtaposes sheared blue-green-grey serpentinite on the west (left) Fault-Related rocks block against brown sheared mudstone and siltstone on the east (right) block. Fault dips approximately 60° northeast; e sheared serpentinite at Nelson Creek approximately 50 cm from the We describe the geologic and geochemical rock primary slip surface with interlaced slickensided surfaces. Card is properties of fault-related rocks in the SAF at *3km 16 cm long for scale; f sheared mudstone in the hanging wall or depth in the SAFOD borehole and for comparison, we east block of the fault exposure at Nelson Creek, with numerous interweaving sheared surfaces. The phacoidal surfaces dip moder- present data from three near-SAF surface outcrop ately to the west and exhibit a range of slip vector orientations. localities within serpentinite-bearing rock: San Sim- Approximately 50 m SE of the Nelson Creek site, brecciated and eon, Goat Rock Beach, and Nelson Creek, California highly fractured limestone lies within the fault on the NE block; and g another outcrop of a fault trace approximately 600 m SE of (Figs. 1, 2, 3). We focus on the fine-scale to micro- the Nelson Creek exposure (UTM location 10S, WGS 84, 721282 scale variations in composition, alteration, and tex- E, 39858423 N). Fault dips 53° southwest. Carbonaceous material ture of additional samples from the three distinct occurs within shear zone along the trace of the San Andreas fault fault-related rock type units identified adjacent to, or (SAF). The Pliocene Etchigoin Formation sandstones (Te) lies in the west (right) block of the fault, and unnamed sandstone, within, the actively deforming regions as described in siltstone, with clasts of serpentinite and sheared marble lie in the a previous paper (Figs. 2, 3 and Table 2 of BRADBURY east (left) block. Neither unit is thought to be a petroleum source et al. 2011). rock in the area, and thus the oil seep indicates the fault strand is charged, and allows up-dip migration of liquid hydrocarbons Detailed investigation of the fault-related rocks reveals distinct compositional and structural features systems (BARTH et al. 2013;HIRTH and GUILLOT 2013; that are similar to rock collected from the three out- MOORE and LOCKNER 2013). crop localities and at several other serpentinite- Previous and on-going analyses of whole-rock bearing or serpentinite-derived exposures throughout core suggests the mechanical properties of the SDZ central to northern California (COLEMAN, 1996; and CDZ are likely influenced by: (1) the presence of DIBBLEE, 1971;DICKINSON, 1966;MOORE and RYMER neo-mineralized clay coatings on interconnected 2012;PAGE et al. 1999;RYMER et al. 2003;SHERVAIS fracture surfaces (HOLDSWORTH et al. 2011;JANSSEN et al. 2004, 2011;SIMS 1988, 1990). Observations and et al. 2012;SCHLEICHER et al. 2010, 2012); (2) for- sampling of the surface outcrops provide additional mation of amorphous matter related to syn- insight into the composition and spatial variability of deformational fault lubrication (JANSSEN et al. 2010; the internal structure of the fault and related damage RYBACKI et al. 2010); (3) cataclasis and brittle mi- zones at SAFOD and the potential physical and crofracture, intense shearing and multi-phased veins chemical processes that may occur at the meter to related to episodic deformation events and transient sub-meter scale in the active SAF. fluid flow (BRADBURY et al. 2011;MOORE and RYMER The composition, alteration, and texture of Phase 2012;RYBACKI et al. 2010); (4) pressure solution 3 whole-rock core samples were characterized by creep mechanisms (GRATIER et al. 2011; Holdsworth using petrographic thin-section studies, X-ray dif- et al. 2011;MITTEMPHERGER et al. 2011); and (5) fraction (XRD), X-ray fluorescence (XRF) analyses, presence and/or transformation of frictionally weak scanning electron microscope (SEM) backscatter minerals within clay gouge such as saponite, talc, imaging, total organic carbon (TOC) measurements, and/or serpentine (CARPENTER et al. 2011;LOCKNER determination of loss on ignition (LOI) values, and Table 1 Summary and comparison of mineralogical composition, alteration, and meso-scale to micro-scale textures identified in SAFOD fault rocks and surface outcrops

Core sample depth Lithologic/ Mineralogy/ Meso-scale texture Micro-scale texture Deformation style SEM texture range (m MD) structural unit alteration*

SAFOD 3,187.4–3,192.7 Foliated siltstone/ Si high Block-in-matrix with Brecciated clasts with Brittle and Conglobulated shale with Carbonates pinch-and-swell carbonate and silica distributed clay block-in-matrix Fe, Ti-oxides structure micro-veins [transition Platy clay coated fabric (magnetite) Veins restricted to clasts Cataclasite injected zone–moderate discrete slip Fe-sulfides Clasts moderately to Cataclasite oxides aligned to low strain surfaces LOI: low intensely deformed in fractures zone] CIA: moderate to high Pervasive scaly clay matrix Average Dmod value–

2.35 cm Geophys. Appl. Pure al. et Bradbury K. K. 3,192.7–3,196.3 Black Fault Rock Si moderate Aphanitic, ultra-fine Layered cataclasite Brittle [transition Compacted Calcite veins cataclasite Clay transformation to fault core– cataclasite Fe-oxides Veins cross-cut matrix Discrete slip surfaces high strain Platy clay coated Fe-sulfides and parallel to foliation Carbonate micro-veins zone] discrete slip LOI: moderate to high surfaces Oxides aligned in surfaces CIA: moderate Injection of black fractures Open to healed carbonaceous material microfractures Average Dmod value– Carbonaceous 2.21 cm matter 3,197–3,198 SDZ Gouge Saponite Scaly clay fabric (non- Anastomosing slip Distributed, Conglobulated Serpentine (Lizardite) cohesive) with rounded surfaces locally brittle clay Cr-Spinel porphyrolcasts Sigmoidal shears *S-C [fault core–low Anastomosing Garnet Veins restricted to clasts fabric to high strain fabric Calcite veins excepting near Micro-veins in clasts zone] Fe-, Mg-oxides bounding slip surfaces Clay-rimmed clasts Pyrite framboids Slickenlined platy clay Reworked cataclasite Organic carbon surfaces clasts LOI: high Phacoidal clasts CIA: high 3,296–3,299 CDZ Gouge Saponite Scaly clay fabric (non- Anastomosing slip Distributed, Conglobulated Serpentine (Lizardite) cohesive) with rounded surfaces locally brittle clay Cr-spinel porphyroclasts Sigmoidal shears *S-C [fault core–low Anastomosing Garnet Veins restricted to clasts fabric to high strain fabric Calcite except near bounding Micro-veins in clasts zone] Magnetite slip surfaces Clay-rimmed clasts Organic carbon Slickenlined platy clay Reworked cataclasite LOI: high surfaces clasts CIA: moderate Phacoidal clasts Table 1 continued

Core sample depth Lithologic/ Mineralogy/ Meso-scale texture Micro-scale texture Deformation style SEM texture range (m MD) structural unit alteration*

3,299.1–3,312.1 Foliated siltstone/ Si low Block-in-matrix with Anastomosing fabric near Brittle and n/a shale with MG-oxides pinch-and-swell contact distributed block-in-matrix Fe-sulfides structure Reworked cataclasite and [transition to fabric Caclite Abundant veins serpentine clasts damage zone– LOI: low to high Clasts moderately to locally low strain zone] CIA: high weakly deformed Micro-laminae visible Blocks of variable Silty sandstone clasts

lithology (up to C1m) with angular grains and Interactions Fluid-Rock and Processes Deformation for Evidence Scaly clay matrix locally absent to weak Breccia zones deformation Clay gouge in shear zones Abundant calcite, locally locally fibrous as thick to thin Average Dmod value–3.9 veinlets cm

Surface San Simeon, CA Sheared melange Si high Block-in-Matrix Fabric Anastomosing Fabric Brittle and Conglobulated outcrop (block-in- Serpentine (Antigorite (cohesive) *Pinch- S-C Fabric distributed clay analogs matrix fabric) ± Lizardite) and-Swell to Rounded Micro-Veins in Clasts Anastomosing Saponite Clasts Clay-Rimmed Clasts fabric Chlorite Gouge zones with scaly Reworked Cataclasite Montmorillonite clay fabric with Clasts rounded to sheared serpentinite clasts Goat Rock Beach, Sheared melange Saponite Block-in-Matrix (non- Anastomosing Fabric Distributed Conglobulated CA (block-in- Serpentine (Lizardite cohesive) with S-C Fabric Micro-Veins clay matrix fabric) ± Chrysotile) Rounded Clasts in Clasts Anastomosing Actinolite Gouge zones with scaly Clay-Rimmed Clasts fabric Cr-spinel clay fabric with Reworked Cataclasite rounded serpentinite Clasts clasts Nelson Creek, CA Sheared Serpentine (Lizardite Block-in-Matrix Anastomosing Fabric Brittle and Conglobulated serpentinite, ± Chrysotile) (cohesive to non- S-C Fabric distributed clay foliated Carbon cohesive) with Micro-Veins in Clasts Anastomosing siltstone/shale Saponite Rounded Clasts Reworked Cataclasite fabric Cr-spinel Foliated textures Clasts CIA chemical index of alteration, LOI loss on ignition K. K. Bradbury et al. Pure Appl. Geophys. calculations of the Chemical Index of Alteration At the meso-scale, the average clast diameter is 2.35 (NESBITT and YOUNG 1982; Table 1; Supplemental cm (Table 1; method of MEDLEY and GOODMAN 1994) data files). Loose chips or slices of core *1/8–1/3 of for the 10 m that it is cored southwest of the SDZ. the core-diameter (d = 10 cm) between 3,185 and The matrix surrounding the clasts is pervasively 3,315 m MD were collected from the core; however, fractured, with local variations in the degree of due to timing and/or restrictions related to the pro- shearing. Intensely fractured zones are characterized cessing and distribution of the SAFOD Phase 3 by highly polished black slip surfaces (Fig. 2i) or whole-rock core, systematic sampling at regular, zones of irregular black staining (Fig. 2b). This unit closely-spaced intervals has not been feasible. We of moderate to intensely damaged rock also correlates report all depths for core samples as the measured to a broad ([200 m) region of low velocity waves drilling depth (m MD) along the deviated borehole. measured at SAFOD (ELLSWORTH et al. 2011;JEPPSON We present results from the analyses of 25 total et al. 2010;ZOBACK et al. 2010). samples from the SAFOD near-fault environment, Northeast of the CDZ contact, a 3-m wide zone of including: (1) foliated siltstone-shale with pinch-and- similar damaged rock occurs and consists of sheared swell block-in-matrix fabric (SILVER and BEUTNER siltstone/mudstone with block-in-matrix fabric and 1980); (2) black cataclasite to ultracataclasite fault- pinch-and-swell structures, and is pervasively frac- related rock; and (3) serpentinite-bearing and ser- tured and dissected by cm-wide shear zones and pentinite-derived phyllosilicate-rich fault gouge abundant mm- to cm-thick calcite veins (Fig. 2h, i). (Table 1). Here, several veins exhibit fibrous calcite growth We also examine micro-scale composition, alter- (Fig. 2h). In general, damage intensity decreases to ation patterns, and textures using petrographic thin- the northeast of this contact, as indicated by clast or sections, SEM, and XRD of *30 surface samples block size, relative fracture intensity, and the number collected at the three different surface outcrops, and of veinlets within clasts (see Phase 3 Core Photo compare these results to analyses of rocks sampled at Atlas, http://www.earthscope.org; BRADBURY et al. SAFOD (Figs. 1, 2, 3, 4, 5, 6, 7; Table 1). We doc- 2011); however, localized zones of intense fracturing ument the presence of three fault-related rock types in and shear are observed to the end of Phase 3 core at the core, and we suggest many of the samples col- *3,312.7 m MD (Fig. 2). The meso-scale average lected at the three surface outcrops discussed in this clast diameter for block-in-matrix units increases to paper are compositionally and texturally analogous to *4 cm (Table 1) within the first 10 m to the north- samples collected at depth at SAFOD, and thus, east of the CDZ slip surface, with a few 1–2 m blocks provide insight into scaling relationships of fault zone of siltstone and sandstone that exhibit only minor deformation and material properties and the 3D damage and a significant decrease in overall fracture structural and permeability architecture of the SAF intensity. system surrounding SAFOD. At the microscale, the block-in-matrix rock unit is characterized by sedimentary lithic fragments, reworked cataclasite (Fig. 4a), and/or clay-clast 3. Results aggregates (see BOUTAREAUD et al. 2008, 2010) embedded within fine-grained, sheared cataclasite 3.1. Foliated Siltstone-Shale with Block-in-Matrix bands with internal layering alternating with foliated Fabric scaly clay-rich rocks, fine-grained cataclasite, and/or Southwest of the SDZ, a foliated siltstone-shale breccia. Within the cataclasite bands, thin seams of unit with a scaly clay fabric and block-in-matrix irregular and discontinuous opaque oxides, primarily texture (Fig. 2a, b) was sampled from *3,187.4 to magnetite, infill fractures parallel to the foliation 3,192.7 m MD in SAFOD Phase 3 core (Table 1). (Fig. 4a). Injection-like structures filled with distinct This material contains intensely damaged clasts that black-ultrafine matrix, cataclasite, and clay are commonly exhibit distorted to stretched and pinch- oriented at sharp boundaries perpendicular or oblique and-swell shapes with numerous veinlets (Fig. 2a, h). to the main cataclasite fabric (Fig. 4a). Multiple Evidence for Deformation Processes and Fluid-Rock Interactions

a 1 mm Core Depth 3 (m MD) Al2O3 FeO* MgO P2O5 LOI (%) SiO2TiO2 CaO Na2O K2O 3,185.00 2 1

3,187.50 3187.5 m b

3,190.00

3,192.50 3193.9 m 0.5 mm c

3,195.00

3,197.50 3194.8 m 0.5 mm d

3,200.00 0.10 0.30 0.50 0.70 0.90 1.10 0.00 5.00 15.00 20.00 30.00 0.00 2.00 3.00 4.00 5.00 0.00 3.00 4.00 5.00 10.00 25.00 1.00 1.00 2.00 90.00 15.00 4.00 5.00 5.00 10.00 15.00 20.00 0.00 5.00 10.00 20.00 25.00 0.00 0.10 0.40 0.50 5.00 10.00 15.00 20.00 25.00 30.00 40.00 50.00 60.00 70.00 80.00 0.00 1.00 2.00 3.00 6.00 7.00 8.00 0.20 0.30

Sample Legend 3197.7 m 0.5 mm BIM Rocks Clasts Black Fault Rocks SDZ

Figure 4 XRF-ICPMS Data and Photomicrographs of textures of deformed rocks within Phase 3 SAFOD core Hole G, Runs 1–3, which intersected the SDZ (highlighted in red): a Brecciated claystone and cataclasite with flow-like and injection-like morphologies (view is plane polarized light); fractures are filled with opaque groundmass that form boundaries parallel to cataclasite foliation direction and connect to multiple high-angle to perpendicular zones of injected catacalsite at 3187.5 m MD. Brecciated textures and surrounding carbonate mineralization indicate repeated opening, sealing, and deformation. Multiple generations of cataclasite and reworked ultracataclasite are also observed to extend into the injection-like fractures (as shown by white arrows and labeled 1 to 3 from youngest to oldest, respectively) and associated with distinct alteration where the lighter gray clay and carbonates are inferred to be older as they are injected by the darker, younger cataclasite/ ultracataclasite; b Multiple episodes of slip produce alternating fine to ultrafine layers of cataclasite and ultracataclasite of the black fault- related rock at 3,193.9 m MD suggest repeated slip has created localized slip surfaces. Multiple generations of dilational, fine carbonate and quartz veins offset the groundmass, older veins at high-angles, and shear planes. Spherules of pyrite are present within the darker bands of ultracatalasite near the center of the photograph; c Magnified view of the vein morphology in polarized light, providing evidence for multiple fluid phases of varying fluid chemistry at 3,194.8 m where black opaque mineral in the center of calcite-filled fracture is pyrite, suggesting a late phase reducing fluid pulse; d rounded clasts of sedimentary lithics and serpentinite entrained within SDZ fault-gouge at 3,197.7 m (view is polarized light with gypsum plate inserted). Flow-like textures surround clasts, and numerous clasts exhibit altered rinds or attached gouge material and/or fractures, and many veins exhibit distorted morphologies (dashed line). Features that may be indicative of slow, recurrent aseismic slip with fluctuations in fluid interactions. Major element whole-rock geochemistry (XRF/ICPMS) analyses conducted at Weber State University SEM Lab generations of cataclasite and brecciation are relative increases in Fe-rich sulfides and Mg-oxides observed with varying degrees of alteration (Fig. 4a). disseminated throughout the matrix and locally seen High-resolution SEM images show shear zones are at as fracture infillings (Table 1; also refer to BRADLEY high-angles to the foliation fabric of the matrix and et al. 2011). contain well-developed platy clay infilling (Fig. 8a, The XRD results indicate Mg-rich palygorskite b). The clast aggregates consist of conglobulated occurs locally within the fine-grained matrix of (rounded) clay and/or reworked cataclasite grains that SAFOD samples (Table 1; refer to BRADBURY et al. appear to have rotated and flowed between bounding 2011). Results for 12 samples with XRF-ICPMS fracture/foliation surfaces, and may contain continu- analyses show Si concentrations are high southwest ous to discontinuous rims of surrounding reworked of the SDZ, low within the SDZ and CDZ, moderate and/or altered gouge matrix (Fig. 8d). Quartz, feld- to high adjacent to the CDZ boundary, and decrease spar, smectitic clays, and magnetite are the primary to moderate values east of the CDZ (Figs. 4, 5). mineralogical constituents of this rock unit, with Southwest of the SDZ boundary, the block-in-matrix K. K. Bradbury et al. Pure Appl. Geophys.

a

Core Depth (m MD) Al2O3 FeO* MgO P2O5 LOI (%) SiO2TiO2 CaO Na2O K2O 3,295.00 3298.4 m 0.5 mm

3,297.50 b

3,300.00

3,302.50 3299.06 m 0.5 mm 3,305.00 c 3,307.50

3,310.00

3,312.50 3299.9 m 0.5 mm

3,315.00 15.00 20.00 10.00 15.00 10.00 15.00 20.00 25.00 15.00 10.00 30.00 50.00 20.00 25.00 30.00 10.00 20.00 25.00 40.00 60.00 70.00 80.00 90.00 0.00 5.00 0.20 0.40 0.00 5.00 0.00 1.00 3.00 4.00 2.00 0.00 0.10 0.30 0.50 5.00 2.00 5.00 0.00 1.00 3.00 4.00 5.00 5.00 1.00 3.00 5.00 6.00 7.00 8.00 0.00 2.00 4.00 0.30 0.50 0.70 0.90 0.10 1.10

Sample Legend BIM Rocks Clasts Black Fault Rocks SDZ

Figure 5 XRF-ICPMS Data for Phase 3 SAFOD core Hole G, Runs 4–6, which intersected the CDZ (highlighted in red): a CDZ fault gouge at 3288.4 m MD exhibits scaly fabric at the microscale. Note the deep red Cr-spinel grain in the lower left corner of the photo has silica mineralization attached to the fractured surfaces and infilling the microveinlets; b CDZ fault gouge at 3,299.08 m MD includes a multi-layered carbonate vein that runs parallel to foliation and connects to multiple high-angle veins oriented perpendicular to fracture (see MITTEMPERGHER et al. 2011 for comparison). Pieces of surrounding matrix occur within veinlet (white circles), and also suggest repeated dilation, healing, and deformation within the CDZ; and c A clast (Core G-4-6) within the block-in-matrix structural unit (BIM rock) adjacent to the CDZ at 3299.9 m MD contains darker spherical stains of organic carbon that contribute to the relatively high TOC values (Table 2) adjacent to and within the fault zone. Major element whole-rock geochemistry from XRF/ICPMS analyses conducted at Weber State University SEM Lab rocks are high in silica and locally high in calcite Chemical index of alteration (likely associated with intraclast veining), with ¼ CIA ¼ Al2O3=ðAl2O3 þ Na2O ð1Þ titanium oxide and iron oxide also present in þ K2O þ CaOÞ100 moderate amounts (Table 1; Fig. 4). Total organic carbon (TOC) measurements southwest of the SDZ, Typically, CIA values for clay are in the range of between 3,187.4 and 3,189.3 m MD, averages *1.2 75–100, with muscovite *75, feldspars *50, and % (Table 2), with higher values associated with black fresh igneous rocks *30–55 (NESBITT and YOUNG irregular staining (Fig. 2). Northeast of the CDZ, 1982). However, the presence of carbonates (e.g. between 3,299 and 3,301.5 m MD, average TOC veinlets, cement) may lead to low and unreliable CIA increases to *1.4 % and hydrocarbons are observed values (BAHLBERG and DOBRZINSKI 2011). The CIA locally at the micro-scale (Fig. 5c). Values of volatile values are typically moderate to high for the phyl- elements measured by loss on ignition (LOI) are low losilicate-rich material in the block-in-matrix units relative to rocks to the immediate east and to the SDZ and in the SDZ and CDZ (Tables 1, 2) due to the fault gouge (Table 1; Fig. 4). abundance of clays within the gouge matrix. In We apply the Chemical Index of Alteration general, samples that include fragments, or that are (NESBITT and YOUNG 1982) to test for any observable entirely composed of clasts with carbonate veinlets patterns in geochemical alteration of SAFOD sam- show highly variable compositions (Table 2) and ples, particularly across the active fault zone, where lower CIA Index values (Table 2). Locally, the the ratios of major elemental oxides from XRF presence of calcite veins may increase the LOI for analyses are used in the following equation: clasts relative to the surrounding phyllosilicate Evidence for Deformation Processes and Fluid-Rock Interactions

Table 2

Percent Total Organic Carbon (TOC) and application of the Chemical Index of Alteration (CIA after NESBITT and YOUNG 1982) determined from whole-rock geochemical analyses on SAFOD Phase 3 core fault-related rocks

Sample depth (m) Structural unit TOC CIA (%)

3,187.4 Foliated siltstone/shale with block in matrix fabric 1.5 73.5 3,189.3 Foliated siltstone/shale with block in matrix fabric 0.9 75.0 3,193 Foliated siltstone/shale with block in matrix fabric 0.5 na 3,193.6 Black Fault Rock 1.1 55.0 3,193.9 Black Fault Rock 0.4 63.2 3,298.4 CDZ Fault Gouge 0.4 48.9 3,299.1 Contact with CDZ/Foliated siltstone/shale with block in matrix fabric 1.6 52.4 3,299.9 Foliated siltstone/shale with block in matrix fabric 1.9 22.1 3,301.4 Foliated siltstone/shale with block in matrix fabric 1.5 25.1 3,303.6 Foliated siltstone/shale with block in matrix fabric 0.8 65.9 3,310.4 Foliated siltstone/shale with block in matrix fabric 0.9 71.4 3,312.1 Foliated siltstone/shale with block in matrix fabric 0.7 67.4 Sample preparation conducted at USU with XRF-ICPMS analyses run at Washington State University (WSU). Total organic carbon was measured at the University of New Mexico Analytical Geochemical Laboratory matrix, in addition to several gouge samples analyzed thick) carbonate veins are both parallel and at high from the SDZ and CDZ (Fig. 4). angles to the ultrafine comminuted material, and may crosscut older vein systems (Fig. 5). An increase in the abundance of magnetite is observed at the micro- 3.2. Black Ultrafine-Grained Cataclasite scale in both the groundmass and concentrated within 3,192.7–3,193.6 m MD fracture fillings (BRADBURY et al. 2011). Silica- Near the southwestern edge of the SDZ, ultrafine, concentrations are moderate within the few samples cohesive black fault-related rocks are present from tested; this reflects local increases in Na-, Ti-, Al-, 3,192.7–3,196.3 m MD (Figs. 2c, 5c). At the meso- and Fe-oxides (Fig. 4). The TOC values show an scale, this distinctive black rock appears massive to overall decreasing trend towards the SDZ boundary locally finely layered, with mm- to cm-thick layers of with values ranging from *0.4 to 1.1 %; however, brown to gray-black cataclasite and is characterized because black carbon-rich zones or staining parallel in thin-section by alternating fine laminations of to a fractures were observed in numerous places cataclasite, variably altered clay, and hydrofracture throughout the core, more systematic sampling is veins, with slight changes in grain size between necessary to better define the nature and distribution layers of cataclasite (Figs. 2b, 5c). The meso-scale of carbon-rich material. Calculated CIA values are average clast diameter of 2.2 cm within the black moderate for this unit and are slightly lower than the fault-related rock unit is the smallest for all units block-in-matrix unit, and higher in comparison to measured (Table 1). Discrete slip surfaces offset the samples from the CDZ (Tables 1, 2). Values for multi-layered zones of fine to ultrafine-grained LOI % are moderate and appear to increase towards cataclasite (Fig. 5). Veins filled with calcite and the SDZ (Fig. 4). locally pyrites are parallel to or at high-angles to the foliation direction within the fine-grained ground- 3.3. Southwest Deforming Zone (SDZ) and Central mass (Fig. 5c). Several veins (mm-thick to cm-thick) Deforming Zone (CDZ) contain a visible central fracture interface filled with carbonates, Fe-sulfides and Mg-oxides (Fig. 4c), The SDZ is sampled in the core from *3,197 to suggesting that cyclic pulses of fluids with differing 3,198 m MD core depth, and the CDZ from *3,296 redox states occurred in the fault zone. Pyrite to *3,299 m MD core depth, corresponding to the framboids are also present as isolated masses within low-velocity zone (Fig. 2d, e) reported by ZOBACK the gouge matrix (Fig. 8c). Numerous thin (B1 mm- et al. (2010) in the borehole geophysical logs at K. K. Bradbury et al. Pure Appl. Geophys.

*3,192 and *3,302 m MD, respectively. The rocks values typically associated with unaltered protolith of the SDZ and CDZ exhibit a non-cohesive scaly and/or mafic rock compositions (NESBITT and YOUNG clay fabric (VANNUCCHI et al. 2003) enveloping 1982). The XRF analyses of samples in both the SDZ rounded to elongate clasts of predominately serpent- and CDZ show fault gouge has higher FeO and MgO inite, clay, quartz, reworked cataclasite, or fine- concentrations than samples in the surrounding rocks grained altered sedimentary or volcanic lithic clasts (Figs. 4, 5). The Ni-, Ti-, and Cr-oxide values are also of mixed composition (Figs. 2d, g, 4, 5, and 6c, e). relatively high within clasts entrained in the fault The complex array of anastomosing surfaces gouge and locally as individual grains within the within the scaly clay fabric displays textural self- matrix. Cr-spinel and andradite garnet were identified similarity on all scales. Loose fragments of fault in several thin-sections (MOORE and RYMER 2009, gouge from the core show pervasive polished and 2012) and in additional samples by SEM and XRD striated surfaces (Fig. 3;SCHLEICHER et al. 2006; analyses (BRADBURY et al. 2011; Figs. 4, 5, 8, 9; 2010). Petrographic observations (Figs. 4d, 5a, b and Tables 1 and 2). In the CDZ, one sample measure- 6c, e) reveal anastomosing phyllosilicate-rich seams ment shows a TOC value of 0.4 (Table 2); however, surround interlayers of fine-grained cataclasite, due to the highly localized nature of black carbona- rotated or sheared clasts and reworked cataclasite ceous material along fractures, additional testing is fragments, and/or carbonaceous material. required to delineate any sort of pattern. The LOI The XRD analyses show the fault gouge matrix is values are higher in the SDZ and CDZ relative to predominately composed of saponite with lizardite ± surrounding rocks with the highest values present chrysotile and magnetite (BRADBURY et al. 2011; within the CDZ fault gouge material (Figs. 4, 5) and LOCKNER et al. 2011;MOORE and RYMER 2012). Many support the presence of either organic carbon/hydro- clasts within the matrix are mantled by an outer rim carbons (BALL 1964) or significant hydration and of clay or contain altered material surrounding the alteration of the host-rock (SCHULZ and EVANS 2000). edges (Figs. 4d, 5a, b and 8e, f). Localized zones of cataclasite and microbreccia that are in turn, sur- 3.4. Surface Outcrop Analogs rounded by more competent elongated and rotated clasts mantled with clay or reworked cataclasite with We examined numerous surface outcrops along apparent flow features (Figs. 4d, 5a, b). the San Andreas Fault system and within exposed and Calcite veins are generally restricted to the sheared lenses of faulted serpentinite-bearing matrix isolated clasts or blocks of serpentinite, with the me´langes and/or sheared mixed serpentinites and exception of the main shear zones. If present within sedimentary rocks (Figs. 3, 6, 7, 9) from central to fault gouge, they are commonly parallel or perpen- northern California (Fig. 1) to further evaluate the dicular to the foliation direction (Fig. 2c, d, h). At the nature of faulted serpentinites and to provide context micro-scale, the calcite veins may appear curvilinear for the shear zones sampled in Phase 3 core. The three or slightly disrupted and fragmented by reworking outcrop site localities we discuss here include: (1) (Figs. 4d, 8d). Pyrite occurs as isolated framboids and near the San Simeon Beach where the San Gregorio- spherules in the fine-grained gouge matrix and as Hosgri fault system cuts the serpentinite-bearing euhedral crystals adjacent to and lining numerous Franciscan Formation (BAILEY et al. 1964;COWAN fracture surfaces (Fig. 8c–f). Distinct high-pressure 1978;HSU¨ 1969;SINGLETON and CLOOS 2012); (2) Goat and/or temperature minerals in the matrix and/or Rock Beach, where a shear zone within the Franciscan clasts include Cr-rich spinel, Ti-sphene, and andradite me´lange is exposed east of an offshore active strand of garnet (Table 1; Fig. 6c). the San Andreas fault (BLAKE et al. 2002); and (3) We apply the CIA calculation to the whole-rock Nelson Creek, about 2 km north of the SAFOD geochemical data as a proxy for alteration and/or intersection with the SAF in a serpentinite-bearing hydration within the SDZ and CDZ relative to the shear zone with adjacent foliated siltstones and surrounding rock samples (Table 2; Figs. 4, 5). Indi- mudstones of the Great Valley Formation (MOORE vidual clasts within the fault gouge show low CIA and RYMER 2012) and in another exposure of faulted Evidence for Deformation Processes and Fluid-Rock Interactions

a b

3193.6 m 1 mm San Simeon Outcrop 1 mm c d

Cr

3197.9 m 1 mm San Simeon Outcrop 1 mm e f

3197.7 m 1 mm Goat Rock Outcrop 1 mm

Figure 6 Thin-section photographs of sheared rocks in SAFOD Phase 3 samples shown in the left column versus analog materials shown on the right and sampled from exhumed exposures of Franciscan me´lange, central to northern California. Strong similarities exist and suggest SAFOD fault-related rocks have experienced similar processes during deformation and fluid-rock interactions, and/or the SAFOD rocks sampled in the shear zones contain Franciscan protolith. a Black fault-related rock adjacent to the SDZ from SAFOD Phase 3 Core at 3,193.6 m MD; b Black fine-grained rock within exposure of Franciscan me´lange sampled near San Simeon, CA; c Fault gouge associated with SDZ from SAFOD Phase 3 Core 3,197.9 m MD; note alteration rind surrounds clast and contains coating of surrounding groundmass suggesting rotation of clast; note opaque to dark-red, high-relief grain near base of picture (Cr-Spinel (Cr); Also refer to Figs. 3–7 of MOORE and RYMER 2012); d Sheared fault gouge from near San Simeon that contains clasts of recycled cataclasite and distinct grains of dark red Cr-spinel, similar in both composition and texture to the SAFOD fault gouge of c; e Samples fault gouge at the micro-scale illustrate the distinct pervasively foliated texture associated with the SDZ and CDZ of SAFOD (shown is the SDZ at 3,197.7 m MD); carbon analyses suggest that the isolated and darker colored groundmass may be infused with hydrocarbons); and f Sheared phyllosilicate-rich rocks within the Franciscan Formation near Goat Rock, northern CA are nearly identical to the textures shown in (e) K. K. Bradbury et al. Pure Appl. Geophys.

a d g

b e h

c f i

4 mm

Figure 7 Optical photomicrographs of samples from the Nelson Creek outcrop. All samples are from the transect shown in Fig. 3d. Images a–e are cross-polarized light images, and f–i are plane polarized light images: a sheared serpentinite with original brown hornblende and biotite and white-grey sheared alteration products of lizardite and chrysotile; b foliated serpentinite cut by thin horizontal shear surfaces; c altered serpentinite cut by fibrous antigorite-chrysotile vein (A); d foliated serpentinite with pyrite grains (black grains); e evidence for pore-fluid pressure lies in the presence of calcite veins, V, that cut the cataclasite; f clay-rich sample from the sheared serpentinite west of the fault contact at the Nelson Creek 1 outcrop, with dark hydrocarbon-rich staining; g fine-grained clay-rich sheared rock of the slip surface (S) from Nelson Creek 1 site; h clast of fine-scale cataclasite (C) cut by foliated hydrocarbon-rich cataclasite, cut by thin veins (V) from the Nelson Creek 2 site; and i repeated slip and development of foliated cataclasite in the hydrocarbon-rich fault dominated rocks in the fault strand fault is shown by light clast of foliated cataclasite, with horizontal banding (FC1) as a clast within dark second phase of foliated cataclasite (FC) from the Nelson Creek site shown in Fig. 3f sedimentary rocks dissected by a hydrocarbon-bear- by a serpentinite-bearing block-in-matrix me´lange. ing fault zone (Figs. 1, 3). Comparison of the fault- From the meso-scale to micro-scale, the exhumed related rocks from SAFOD to the samples collected at black rocks are similar in composition and texture to the surface outcrops indicate that the rocks are the black fault-related rock unit sampled in SAFOD compositionally and texturally analogous. For exam- Phase 3 Core (Figs. 2c, 5c, 6; Table 1), whereas the ple, near San Simeon, CA (Fig. 3), black ultrafine surrounding foliated serpentinite-bearing scaly clay rocks exhibit laminated to contorted morphologies in matrix compares to the fault gouge of the SDZ and narrow (cm-wide to m-wide) shear zones surrounded CDZ (Figs. 2d–g, 3, 6). Evidence for Deformation Processes and Fluid-Rock Interactions

Fault-related rocks from the SDZ (Figs. 2d, e, 4d) interweaving of slip surfaces is common with the 3 and CDZ (Figs. 2f, g, 5a, b) exhibit evidence for m east of the fault (Fig. 3f), and multiple orientations brittle and ductile deformation at the micro-scale. of slip vectors are recorded on slip surfaces, where Evidence for brittle deformation includes the pre- narrow zones of indurated cataclasite are faulted next sence of broken grains, reworked cataclasite to clay-coated fault surfaces. The delicate nature of fragments, and discrete microfractures (Figs. 4d, 5a, the clay-coated surfaces may have formed in the 2004 b, 6, 8). Flow-like morphologies of the faulted rocks, Parkfield earthquake (MOORE and RYMER 2012), and block-in-matrix fabrics, and S-C fabrics in the fault the multiple slip vectors suggest that surface ruptures gouge all support an interpretation of distributed have complex slip geometries. To the south, the fault shearing within these zones (Fig. 6d, e). Samples cuts sedimentary rocks where multiple generations of from the CDZ (Fig. 6e) and Goat Rock Beach cataclastically deformed clasts lie in a clayey-hydro- (Fig. 6f) both contain complex anastomosing net- carbon rich matrix (Fig. 3g). At the micro-scale works of slip surfaces. This distinct fabric is also (Fig. 7), the serpentinite-rich unit is foliated and identified in SEM images of thin-sections for samples sheared and cut by veins of lizardite and fibrous from the SDZ and Goat Rock Beach (Fig. 9a–c). chrysotile, calcite, with localized pyrite mineraliza- Sheared me´lange samples from San Simeon and tion (Fig. 7a–e). Fine-grained sedimentary rocks Goat Rock (Fig. 3a–c) occur in tabular zones that are contain multiple-layers of cataclasite and clay gouge on the order of several meters thick and exhibit with irregular dark hydrocarbon-rich staining that are deformation features uniquely similar to those cut by thin veinlets of calcite (Fig. 7f–h). observed in the SDZ and CDZ rock samples from SAFOD Phase 3 core (BRADBURY et al. 2011;GRATIER et al. 2011;HOLDSWORTH et al. 2011). At the San 4. Discussion Simeon site, the rocks appear more cohesive relative to the non-cohesive gouge sampled in the SAFOD The mineralogical, textural, geochemical, and core; however, at finer scales, the structures are geophysical signatures described here and by others analogous (compare Figs. 6, 8, 9). The sheared (BRADBURY et al. 2011;CARPENTER et al. 2011; melange matrix from near San Simeon also contain LOCKNER et al., 2011;ZOBACK et al. 2010), charac- numerous clasts of recycled cataclasite (Fig. 5e), terize the SDZ and CDZ fault strands separated by calcite veins, pyrite framboids and spherules, and over 100 m in the SAFOD borehole. These fault distinct grains of dark red Cr-spinel, with similar strands are possibly linked at depth (BRADBURY et al. mineralogies to those observed in both the SDZ and 2009) and extend to the surface (the Nelson Creek CDZ fault gouges (Figs. 6, 8, 9). outcrop of MOORE and RYMER 2012 also described The San Andreas Fault is exposed at the Nelson here), to form a wide zone of deformation charac- Creek surface outcrop, *2.3 km northeast of the terized by an anastomosing network of foliated, SAFOD site (Figs. 1, 3, 7). MOORE and RYMER (2012) phyllosilicate-rich fault rocks within which narrow described this outcrop, show these rocks are textur- slip surfaces are locally separated by blocks or lenses ally and mineralogically similar to those observed in of competent and/or less deformed rocks (including the SAFOD core, and suggest they are linked to the sedimentary and volcanic rocks) in the shallow crust SDZ and CDZ at depth. The Nelson Creek outcrop (BRADBURY et al. 2009). The comparisons presented consists of large blocks of serpentinite entrained here between the composition and texture of the within a non-cohesive, Mg-rich scaly clay gouge SAFOD fault rocks and the San Simeon and Goat (Fig. 3d–g; MOORE and RYMER 2012). At the outcrop Rock outcrops within the Franciscan Formation, and scale, the fault consists of sheared serpentinite west the work by MOORE and RYMER (2012) on the Nelson of the fault contact next to clay-rich sheared rocks to Creek outcrops of the Coast Range Ophiolite, suggest the east. Serpentinite here has a blue-grey cast the processes operating within the SAF during (Fig. 3d, e), and numerous surfaces have striations deformation may be comparable to those that affected with a range of orientations. The cm-scale these near-surface outcrops. Thus, these structures, K. K. Bradbury et al. Pure Appl. Geophys.

a b

3187.5 m 30 µm 3187.5 m 1 mm

c d

3197.4 m 50 µm 3197.7 m 1 mm

e f

3197.9 m 500 µm 3299 m 50 µm Evidence for Deformation Processes and Fluid-Rock Interactions b Figure 8 within the fault gouges have very low CIA values Scanning Electron Microscope images of SAFOD Phase 3 Core: (Table 2) relative to all other samples. These values a neomineralized clay growth (e.g. SCHLEICHLER et al. 2010) within hairline fractures of black fault-related rock at 3,187.5 m MD); may not be representative due to the abundance of b conglobulated textures between localized slip surfaces within the carbonate veins within some of the clast samples black fault-related rock (scalebar represents 1 mm) at 3,187.5 m (BAHLBERG and DOBRZINSKI 2011). Fibrous calcite MD; c pyrite framboid within fine-grained clayey gouge of the SDZ at 3,197.4 m MD; d conglobulated fault gouge with distorted veins northeast of the CDZ and numerous micro- vein patterns of the SDZ near 3,197.7 m MD (scalebar represents 1 veins southwest of the SDZ (Figs. 4, 5; Table 1) mm) may suggest fluid saturation of fault gouge during shear; suggest that localized fluctuations in fluid pressure e rounded, altered, and clay-rimmed clasts embedded within fine- grained groundmass within foliated fault gouge of the SDZ at along the shear zone boundaries. The abundance and 3,197.9 m MD; and f pyrite growth surrounded by isolated irregular clustering of veins, coupled with S- and Fe-oxide zones of carbonaceous material within clay gouge of the CDZ near mineralization, and increases in TOC (Table 2), are 3,299 m MD Note, depths are reported as measured core depths and evidence for potential interactions with reducing require a depth correction for comparison to geophysical log data (see ZOBACK et al. 2010) fluids and/or hydrocarbon–bearing fluids during fracture and fault slip (COBBOLD and RODRIGUES 2007; textures, and mechanisms may form (or exist) along RODRIGUES et al. 2009). significant portions of the SAF, or along transform Fractures observed here may have resulted from faults that reactivate or initiate within serpentinite- seismic slip and co-seismic fluid infiltration in the bearing rocks. main SAF zone in the black- sheared and stained The block-in-matrix rock units adjacent to the fault-related rocks, and include: slip localization, active fault strands identified at SAFOD (ZOBACK cataclasite, and intense brecciation, polished fracture et al. 2010) record an apparent damage asymmetry of surfaces and microfracturing. Evidence for repeated the active fault zone. Rocks that extend for *10 m deformation in these rocks includes the presence of southwest of the SDZ are characterized by smaller multiple layers of cataclasite and reworked catacla- average clast sizes compared to 10 m northeast of the site clasts and numerous cross-cutting vein sets, CDZ (Table 1), suggesting the SDZ may represent a including the clasts with multiple veinlets in the ca- more localized, evolved strand due to the presence taclasite (Fig. 5). Similar black-stained intervals are of multiple generations of cataclasite, brecciation, also present locally in the block-in-matrix unit to the and/or granular flow (CHESTER et al. 1993;SIBSON southwest of the SDZ, but they are not visible within 1977). The CDZ strand may be the more active trace, the cored interval of the adjacent SDZ gouge. The as suggested by measurements of casing deformation black fault rock and injection-like structures in the within the fault zone that show that creep is more SAFOD core may also represent material generated pronounced in the CDZ relative to the SDZ (JEPPSON during brittle failure from rupture elsewhere along et al. 2010;ZOBACK et al. 2010). the fault that has fluidized and become subsequently Directly southwest of the SDZ, microstructures entrained into the creeping portion of the fault (e.g., and fabric within the block-in-matrix and black fault- DAVIES et al. 2012). related rock units display evidence for localized and The TOC analyses indicate that at least some of distributed deformation (Table 1; Figs. 6, 7, 8, 9). the black-stained rocks that occur adjacent to, and Injection of granular material and fine particles into within, the SDZ and CDZ are relatively rich in surrounding matrix coupled with brecciation (Fig. 4a) organic carbon, and locally reach petroliferous grades may occur in response to increased fluid pressures (Table 2;D.KIRSCHNER written comm., 2011;PETERS and natural hydraulic fracturing (UJIIE et al., 2007). and CASSA 1994), supporting the interpretation that From southwest to northeast across the SAF, fol- some migration of hydrocarbon-bearing fluids, pos- lowing the deformation trend, the alteration pattern as sibly from a Great Valley source, has occurred along estimated by the CIA for the analyzed samples is and into the active fault strands (BRADBURY et al. higher southwest of the SDZ and lower between the 2011). The black staining with injection-like mor- SDZ and CDZ, returning to higher values east of the phology coupled with multi-phase calcite veins CDZ trace (Tables 1 and 2). Several clasts entrained indicates that the mobilization of hydrocarbons and K. K. Bradbury et al. Pure Appl. Geophys.

a b

1 mm 1 mm c d

50 µm 1 mm

e f

50 µm 50 µm

Figure 9 Scanning Electron Microscope images of exhumed shear zones within the Franciscan Formation: a pervasive foliated fabric exists to the submicron scale in sample in shear zone within Franciscan Formation near Goat Rock, CA; b isolated and altered clasts with varying degrees of roundness appear to ‘‘float’’ within fine to ultrafine-grained groundmass (see also Fig. 3; sample from shear zone within Franciscan Formation near Goat Rock); c alteration and/or clay transformation pattern appears to be associated with an interaction between the fracture system and the phacoidal-shaped clast, suggesting extensive fluid-rock reactions at this scale (sample from near Goat Rock, CA); d conglobulated texture and alteration of clasts within sheared rock of the Franciscan Formation near San Simeon, CA; e pyrite mineralization parallel to foliation direction and fracture surface, shear zone within Franciscan Formation, San Simeon, CA; and f spherule of pyrite within groundmass of shear zone within Franciscan Formation, near San Simeon, CA Evidence for Deformation Processes and Fluid-Rock Interactions other fluids (liquids and/or gas) may be related to to phyllosilicates, and thus enhance fault zone hydrofracture and transient changes in pore fluid weakening mechanisms (KOHLI and ZOBACK 2013; pressure during slip (HOLDSWORTH et al. 2011; OOHASHI et al. 2013). MENEGHINI and MOORE 2007;MITTEMPHERGER et al. Within the shear zones of the SDZ and CDZ 2011; ROWE et al. 2009). The presence of hydrocar- (Figs. 4, 5), the intensity and variability of fabrics bons in the fault zone rocks, the presence of and alteration is high and includes S-C fabrics, carbonaceous residues in SAFOD samples, the strong breccias, multiple generations of cataclasite and petroliferous odor observed during handling of this reworked cataclasite, anastomosing phyllosilicate- portion SAFOD core from the CDZ, the shiny black rich surfaces, clay-rimmed clasts, and elevated con- appearance on some fracture surfaces, the carbon and centrations of Fe-Mg oxides and sulfides relative to oxygen isotopic signatures of veinlets from the core the surrounding host rocks. These characteristics (KIRSCHNER et al. 2008), and the reports of hydro- suggest that distributed deformation and/or fluid- carbons in the borehole after Phase II drilling assisted deformation occurs within the fault zone. (HENYEY et al. 2011) indicate elemental carbon ± free Thin-section and SEM images also show that larger hydrocarbons occurs within the SAF at depth. Mud grains or clasts appear to have fines and matrix gas analyses by WEISBERG and ERZINGER (2011) material adhered to their outer surface, suggesting the showed evidence for the presence of hydrocarbon- grains have rotated in an irregular, flow-like process bearing fluids within the SAF and the presence of throughout the gouge matrix (Fig. 8). Clay clast methane in the fault zone at 3,150–3,300 m MD. aggregates (BOUTAREAUD et al. 2010;BOULLIER et al. Natural gas and hydrocarbons were also encountered 2009;COLLETINI et al. 2009), clay-rimmed clast in the saline formation waters at SAFOD in 2005 and coatings, and the clay infillings between clasts or 2006, as discussed in HENYEY et al.(2011). Isotopic fragments are also features associated with fluid- analyses of the mud gases indicate a contribution of related processes and are consistent with textures mantle-derived source of the methane (WEISBERG and observed during distributed deformation, thermal ERZINGER 2011) and standard Rock-Eval pyrolosis of pressurization, and slip (BOUTAREAUD et al. 2010; pre-washed cuttings indicates that the rocks between FAGERENG and SIBSON 2010;ROWE et al. 2009). Geo- 3,182 and 3,300 m contain terriginously derived chemical analyses indicate a sharp increase in Mg- thermally mature organic source rock in the dry to oxide and Fe-oxide as silica content decreases. This wet gas range (D. KIRSCHNER written comm. 2011). trend is due to the abundance of smectite (saponite) in

The Ro (vitrinite reflectance) values from cuttings the scaly clay fault gouge (BRADBURY et al. 2011; analyses range from 0.70 to 1.2, well within the oil LOCKNER et al. 2011;MOORE and RYMER 2010;MOORE generation window and verging to the condensate- and RYMER 2012). Saponite has an extremely low wet gas window from Type III kerogen (D. KIRSCHNER, coefficient of friction (l & 0.15–0.21) in laboratory written comm. 2011). studies and exhibits stable-sliding frictional behavior The carbon/hydrocarbons could significantly (CARPENTER et al. 2009;CARPENTER et al. 2011; influence geochemical alteration and slip processes LOCKNER et al. 2011). Saponite is also commonly a along the SAF at SAFOD and be another key factor in fluid-assisted alteration product of serpentinite contributing to fault zone weakness and slip locali- (BREARLEY 2006; ZOLENSKY et al. 1993) that exhibits zation, due to reduction of the coefficient of friction plastic tenacity when hydrated and is brittle when dry (BARTON and LIEN 1974;KOHLI and ZOBACK 2013; (http://handbookofmineralogy.org). Lizardite ± OOHASHI et al. 2011, 2012, 2013), an associated chrysotile compositions were identified in XRD and increase in pore pressure within the fault zone SEM (BRADBURY et al. 2011;MOORE and RYMER 2007, (ZOBACK et al. 2010), or due to the presence of natural 2012;MORROW et al. 2010;SOLUM et al. 2007), and in gas in the fine-grained gouge. Experimental results thin-section, often in association with magnetite, show that enrichment of carbon in fault zones may pyrite, Ni-oxides, Cr-spinel, and/or garnet within contribute to the development of lubricated and both the SDZ and CDZ fault gouge matrix (Table 1; penetrative slip surfaces during deformation, similar MOORE and RYMER 2012). The distinct mineral K. K. Bradbury et al. Pure Appl. Geophys. assemblage may indicate that fluids play an effective requires a source of fluids at depth and extensive role of mobilizing and concentrating elements from a physio-chemical interactions to bring material from serpentinite-rich host-rock within the fault zone at the depth. Similar fluid-assisted processes of transporting grain and slip-surface scale (MICKLETHWAITE et al. serpentinite-bearing rocks from depth have been 2010). documented in accretionary prism me´lange deposits The CIA index ranges from low to moderate for of the Franciscan Formation (CLOOS 1984) or through the SDZ and CDZ (Figs. 4, 5), but this result may be volumetric expansion associated with serpentiniza- a function of the abundance of Mg-rich and Fe-rich tion (PAGE et al. 1999;SHERVAIS et al. 2011). clay in the gouge matrix rather than Al and the We suggest that the fault-related rocks at SAFOD abundance of relatively unaltered clasts. The LOI are strikingly similar to the serpentinite-bearing fault values may also be used as an indicator for fluid gouge and serpentinite me´langes within the faulted assisted alteration within the fault zone (SCHULZ and Franciscan Formation surface outcrops exposed at Goat EVANS 1998). Results for the samples tested show Rock, San Simeon, and Nelson Creek (Figs. 3, 4, 5, 6, 7, LOI is greatest in the CDZ region (Fig. 5), where a 8, 9; Table 1), and perhaps are charged with a range of positive correlation to higher TOC values is also hydrocarbons (Fig. 3f; Table 2;KIRSCHNER et al. 2008) observed (Table 2). due to enhanced permeability parallel to strike and The spatial variability in composition, alteration reduced permeability perpendicular to the fault strands patterns, and textures spanning multiple scales of (WIERSBERG and ERZINGER 2011). Me´lange fabrics may observation across the SAF in SAFOD supports the form through a variety of depositional and post-lithifi- interpretation of mixed styles of deformation (FAG- cation processes, including tectonic folding and ERENG and SIBSON 2010) or multiple deformation faulting, sedimentary and slope failure processes, and events, and supports the idea of a dynamic structural origin by vertically driven movements related to dia- and permeability architecture. In-situ sampling across piric and/or volume expansion processes (BAILEY et al. the SAF at *3 km depth at SAFOD demonstrates that 1964;FESTA et al. 2010;RAYMOND, 1984;SHERVAIS et al. the fault zone exhibits wide zones of pervasive 2011;SILVER and BEUTNER 1980;VANNUCCHI et al. 2003; deformation (*10–15 m) separated by thin, anasto- WAKABAYASHI and DILEK 2011). Each process is asso- mosing (*1–2 m) foliated gouge zones (SDZ and ciated with distinct rock textures and/or compositions CDZ), numerous discrete slip surfaces (mm-cm thick) that may reflect the style of deformation (COWAN 1985; and a few entrained blocks (*1–2 m in core length) RAYMOND 1984;VANUCCHI et al. 2003). The fact that the that are only weakly deformed. These textures create a SAFOD fault-related rocks appear nearly identical structural fabric that implies deformation processes from the meso-scale to micro-scale to the me´lange related to aseismic creep and stable frictional sliding samples examined at the surface outcrops, suggests (COLLETINI et al. 2009;FAULKNER et al. 2003;REINEN similar processes of origin such as: (1) block-in-matrix et al. 1991) with periodic seismic events. The pre- textures and scaly clay fabrics characterized by S-C sence of saponite coupled with the potential for a surfaces; (2) sheared serpentinite clasts and blocks; (3) periodic influx of hydrocarbons within the active multiple phases and styles of calcite veining (Fig. 6d, shear zones (Fig. 3f) may contribute to the formation e); (4) pyrite mineralization (aligned within interlay- of alteration products and/or foliated textures that ered fractures and as isolated framboids within the promote additional weakening and deformation of the matrix (Figs. 6, 7, 8, 9); (5) fluidized and conglobulated SAF (COLLETINI et al. 2009; 2011; HAINES et al. 2009; textures within structural zones; and (5) alteration-rims LOCKNER et al. 2011;OOHTANI et al., 2013). surrounding numerous clasts entrained within the Based on the presence of serpentinite and higher- gouge or matrix materials (Fig. 7c). temperature mineral assemblages, MOORE and RYMER A range of work has examined portions of the (2012) and LOCKNER et al.(2011) also hypothesize SAFOD core to evaluate the mechanical strength of that serpentinite is channelized up along the fault these rocks. In general, workers agree that the SAF at zone from depth and influences metasomatic reac- the SAFOD site comprises a region of aseismic creep tions within the fault. This buoyancy-driven origin and small magnitude seismic rupture. Hypotheses that Evidence for Deformation Processes and Fluid-Rock Interactions have been proposed to explain the creep and evidence exhumed, faulted serpentinite-bearing rocks provide for possible rupture include: (1) the presence of insight into the mineralogical composition, geo- frictionally weak minerals within clay gouge such as chemical alteration, and rock textures associated with talc, saponite, and/or serpentinite (CARPENTER et al. active aseismic creep and the conditions that influence 2009;LOCKNER et al. 2011); (2) the presence of high deformation and fluid (liquid and/or gas) interactions density smectite-rich coatings (\100 nm thick) on along the fault within the shallow crust. Sheared and/ fracture surfaces interconnected at low angles or altered serpentinite, fine-grained foliated phyllosi- (SCHLEICHER et al. 2010, 2012); (3) pressure-solution licate-rich fault gouge, mineralization, hydrocarbons creep (GRATIER et al. 2011); (4) record of transient and carbonaceous material are all present within the migration of fluids into the fault zone (HICKMAN et al. fault zone at SAFOD, and may explain why aseismic 2004;MITTEMPHERGHER et al. 2011; WANG 2010); (5) creep occurs along the central segment of the SAF. generation of crush-origin psuedotachylytes (amor- Slow slip may develop as a result of pore fluid phous phases) within fault-related rocks (JANSSEN exchange and the alteration of frictional properties of et al. 2010); and (6) development of multi-layered the materials within the fault (KNIPE 1993;RUDNICKI foliated fault gouge fabrics (COLLETINI et al. 2009; and RICE 2006). The SAFOD core exhibits charac- NIEMEIJER et al. 2010). Our analyses of the SAFOD teristics of rocks that have experienced high-pore fluid Phase 3 core fault-related rock show the actively pressures during their development history; however, creeping SAF at *3 km depth records a complex this cannot be definitively attributed to active slip. The interplay of clay and serpentine mineral slip processes inherent foliated textures may contribute to an aseis- and fluid-rock interactions that evolve in both space mic deformation style, because they form an and time (Table 1; Figs. 3, 4, 5, 6, 7, 8, 9). Amongst anastomosing network of weak phyllosilicate-rich the fluids present within the fault are methane and surfaces. The SAFOD core shows that deformation free hydrocarbons, perhaps migrating parallel to the within the active SAF at depth is highly variable fault after the fault captured organic-bearing lozenges spatially and temporally over relatively finite dis- of organic-rich rock (D. KIRSCHNER, pers. Comm., tances and from the meso-scale to the micro-scale. 2011; WIERSBERG and ERZINGER 2011) and was also Direct sampling and analyses of SAFOD core reveals connected to a deeper source of mantle-derived the SAF structure at depth and provides constraints on methane (KENNEDY et al. 1997;KHARAKA et al. 1999; the physical and chemical processes and tectonic WIERSBERG and ERZINGER 2011;WAKABAYASHI and history associated with active SAF deformation. DILEK 2011). Thus the SAFOD site sampled a remarkably complex region of phyllosilicate-rich, serpentine-bearing sheared rock that experienced significant organic-rich and water-rich fluids that REFERENCES altered the deformed rocks over its history. The unique and complex fault composition of structure of ALLEN, C.R. 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(Received September 7, 2013, revised July 2, 2014, accepted July 5, 2014)